Interpretive Summary: Since 1987, annual losses from silverleaf whitefly, Bemisia argentifolii in the United States have exceeded $200 million, with an additional annual loss of 3,000-6,000 jobs. Repeated applications of conventional insecticides are ineffective in suppressing whitefly populations on cotton during the season because of problems with coverage and insecticide resistance. Application of chemical defoliants in July and August is an important component of the mandated short-season production cotton system. Defoliants are useful in causing lead abscission, earlier boll opening and the shedding of young fruiting forms, thus denying late season food sources for overwintering insect pests. The effects of defoliants on Bemisia are poorly known and have not been investigated for whitefly parasitoids. Lethal and sublethal effects of two commonly used defoliants, Def and Dropp, on whitefly and its parasitoids were evaluated. These findings will be useful for reducing the number of insects dispersing to winter crops in the Imperial and Lower Rio Grande Valley and for successfully integrating applied biological methods. This control method will reduce the need for additional insecticide use and reduce associated environmental problems.

Technical Abstract:
Lethal and sublethal effects of two commonly used defoliants, Def and Dropp, on whitefly, Bemisia argentofolii, and its parasitoids, Eretmocerus eremicus and Eretmocerus hyati, were evaluated in laboratory and greenhouse tests. Whitefly eggs and adults were more susceptible to defoliant treatments than larvae. The reduction in feeding sites differentially affected whitefly nymph mortality, depending on instar. Sublethal effects on Def, Dropp or their mixture on whitefly were manifested through reduction of percentage female progeny and the number of eggs deposited per female per day after spraying young nymphs. The timing of application significantly affected parasitoid survival. After defoliant treatments of whitefly nymphs parasitized with early instar E. eremicus larvae, the number of parasitoid female progeny was significantly reduced and their longevity was significantly shorter than those of controls.